The city administration, which closed streets in two Orthodox Jewish neighborhoods to vehicular traffic for 11 hours last Saturday, has decided to reduce the traffic-ban period to five hours this Saturday because the wintery weather is making walking there “very light.” Involved are the main thoroughfares in the Williamsburg and Boro Park sections of Brooklyn, where Orthodox Jews have expressed the desire for as much freedom of movement as possible on the Sabbath. A spokesman for the Mayor’s office said today that there had been no complaints from the drivers of cars and trucks diverted slightly because of the close-off of the two areas.